1. Field of the Invention
The present invention relates to an X-ray diagnostic system, preferably to a system having a two dimensional X-ray detector, in particular, to the system which is capable of producing a tomogram at any slice only by adding projection data of a plurality of frames acquired by multiple exposures of X-rays to a subject, and the system which is capable of not only altering an X-ray incidence angle on the detector for multiple-purpose imaging but also performing imaging at arbitrary enlargement ratios using the two dimensional X-ray detector.
2. Discussion of the Background
At present, such modalities as an X-CT scanner, X-ray tomosynthesis system, magnetic resonance imaging (MRI) system, and diagnostic ultrasound apparatus have been clinically used for acquiring tomographic images of patients.
Of these X-ray tomosynthesis systems were temporarily used in the past because of relatively easier acquisition of tomograms, but advance in X-ray CT scanners and MRI systems have removed them from clinical fields. Recently, however, easy processing of images and other factors in the X-ray tomosynthesis systems have returned them to the spotlight again.
X-ray tomosynthesis systems are categorized into two types of analog and digital. The analog type of X-ray tomosynthesis uses an X-ray film as an X-ray detector, in most cases. With an X-ray tube including an X-ray focus and an X-ray film positioned face to face across a patient intervened therebetween, the tube and film are driven into their relative movement such that X-ray beams irradiated by the tube pass necessarily through an arbitrarily-set section within a subject to be imaged. As loci of the movement, various loci including a linear locus and 8-shaped curve locus can be selected. This multi-exposure of X-rays can blur image pixels in slices other than a desired slice and focus image pixels in only the desired slice, thereby providing its image.
In this analog type system, whenever slices are set, it is required that X-ray films be replaced and the X-ray tube be scanned in mutually changed movement loci. This requires troublesome handling for exchange of the X-ray films, while exposure of patients becomes large. Additionally, patients need to do their breath hold for each scan. Thus, as the number of set slices increases, variance in the breath hold becomes larger, being likely to cause phase shifts resulting in artifacts on tomograms.
To overcome the drawback, there is provided a digital type of X-ray tomosynthesis system, as is proposed by Japanese Patent Laid-open 57-203430. Like the analog type system, the digital-type tomosynthesis system is constructed such that an X-ray tube and digital X-ray detector are relatively moved in mutually opposite directions, the spatial positions of both the tube and the detector are determined, image information coming from the detector is stored in association with each position, and an image at an arbitrary slice is produced from the stored image information. This makes it possible to produce images at any slice from data acquired by only one time of X-ray scan.
As a digital X-ray detector used by the digital X-ray tomosynthesis system, there are known an imaging plate (IP), I.I.-TV unit, indirect-type planar detector having X-rays/light converting layer (such as intensifying paper, ceramic, or scintillator) and light/electric charge converting layer (such as liquid crystal such as TFT, or photodiode), direct-type planar detector converting X-rays to electric charges, or the like.
However, the above analog- and digital-type X-ray tomosynthesis systems still have some shortcomings which have not been unsolved.
First, control for moving the X-ray tube and the detector in synchronized timing is extremely difficult in terms of technical aspects. The accuracy of this synchronization control is very important to image quality of tomograms. Higher accuracy synchronization control requires more complex control mechanisms and control circuits, resulting in an increase in manufacturing costs.
Second, there is a problem of positional shifts of the X-ray tube (i.e., X-ray focus). Since the conventional tomosynthesis systems do not have particular countermeasures in this respect, such positional shifts surely causes artifacts, deteriorating image quality.
Third, there is a problem concerning movement loci made by the X-ray tube. The conventional systems do not take account of any special countermeasure, image artifacts arisen due to such movement loci may reach to unnegligible levels. For example, when a linear orbit is employed, linear artifacts tend to appear distinctly on a tomogram.
The fourth problem is about contrast resolutions. This type of X-ray tomosynthesis system basically depends on a technique by which constitution members present in slices other than an objective slice are made blurred to produce an image of the objective slice. This technique is likely to produce lowered contrast resolutions. The prior digital type systems, which do not employ any particular prevention of this influence, also suffer from the same drawback.
The fifth problem concerns enlargement ratios of images. The conventional tomosynthesis system cannot produce tomograms in which the image enlargement ratio is taken into account for each slice. Thus, for example, when coronal images etc. separately taken at a plurality of slice positions are displayed in animation, differences in enlargement ratios of images become distinct, thereby making interpretation difficult because of deteriorated visibility of lesions to be diagnosed.
As the sixth, there is a situation that acquired image data of slices have not fully been utilized, though they might have some important information still useful for diagnosis. In the case of the foregoing digital-type system, for example, although it is possible to acquire image data of, as an example, a plurality of coronal slices at one time of scanning, the acquired image data are used for only limited purposes such as displaying as-acquired images.
Still, the seventh problem is derived from a situation that the foregoing digital-type system has two limitations. One is a limitation concerning the scan orbit along which the X-ray tube and the detector should be moved in opposite directions with each other, and the other a limitation for data processing by which signals from the detector are required to be distinguishably memorized for each of the moved positions of the detector. Their limitations lead to a problem that degrees of freedom of device allocation in design but also image processing procedures decrease.
Still further, the eighth problem is that the conventional digital-type system has not provided any practical process for obtaining tomograms when both the X-ray tube and the detector are two- or three-dimensionally moved.
On the other hand, as one X-ray diagnostic system, there is known a cardiovascular X-ray diagnostic system comprising an approximately C-shaped arm which has an X-ray source and X-ray detector attached at both the ends. The C-shaped arm is formed such that it can be rotated about a axis supporting the C-shaped arm (supporting-axis rotation), slide along its C-shape guide (sliding rotation), and the like. These various rotation and translation make fluoroscopic radiography possible in different directions and at different angles.
Further, when used in the diagnosis of digestive systems, known is an X-ray diagnostic system where an X-ray tube and X-ray detector are attached to a couch capable of standing up. In this system, X-ray radiography is executed as the tube is moved along a circular-arc orbit or linear orbit parallel with the couch top, thereby providing tomograms.
However, any of the above-explained X-ray systems has limited imaging modes, because the X-ray detector is restrained by the supporting member of the C-shaped arm. For instance, a cardiovascular X-ray diagnostic system comprising a rotating arm as the supporting member is limited to radiography at a given angle or rotational radiography. These limitations decrease degrees of freedom of the system for clinical applications. For example, contrast examination of the coronary artery sometimes requires setting an imaging angle selected from a range from 55 degrees in the superior (head side) direction to 35 degrees in the inferior (feet side) direction. It is impossible for the conventional cardiovascular X-ray system to perform such deeper angle setting, because the detector hits a subject to be imaged or the table top.